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Civil Engineering/Assigning diaphragm in ETABS


QUESTION: What is the purpose of assigning diaphragm in ETABS while modelling an earthquake resisting structure? If we do not assign a diaphragm, what difference would it make to other structural components i.e. beams columns.
I am confused with this..whether I should assign diaphragm or not. people have different opinions.

ANSWER: Dear Sara

Diaphragm is a model of slabs assuming infinite axial stiffness for the slab. You can model the slab with shell type elements and don't use a diaphragm. The main purpose of diaphragm modelling is to avoid complication and increase of stiffness matrix order which make analysis of great structures time consuming. It is up to you as a designer. What you need ? a good accuracy with minimum effort? Use diaphragm then. If you think that there is some source of error to model all nodes of a level on a same diaphragm then avoid it and model the slab as shell element. Don't forget to have all your nodes in same level in your slab model. Otherwise you may loose some accuracy due to miss application of lateral loads to some nodes.


---------- FOLLOW-UP ----------

Please explain how this effects lateral loads? there are different opinions.. Some say that diaphragms are the basic structural systems that transfer seismic forces to lateral load resisting system. If we do not assign diaphragm, we are ignoring this fact and the true effect of seismic forces are not incorporated in the model, hence we will get the under designed sections.
Secondly if we assign diaphragm and model slabs as a shell member, how is it different from assigning diaphragm and modelling slabs as membrane?

Dear Sara

A diaphragm is a slab with infinite axial stiffness which force all nodes in a level moving together when a lateral load is applied on the structure. Ok?

You can say to yourself why modelling my slab with infinite stiffness and you can in every situation model a slab instead of a diaphragm and the slab play the same rule as the diaphragm. think about the axial stiffness of a slab , this stiffness force 2 nodes moving together. Am I right?


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Babak Esmailzadeh Hakimi


I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!


I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

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Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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